1. Field of the Invention
This invention relates generally to a signal processing circuit for a color image pickup system, and more particularly to such a signal processing circuit in which spurious signals in a synthesized luminance signal are suppressed.
2. Description of the Prior Art
Recently, an investigation has been carried out on a color television camera system in which solid state imaging devices such as a CCD (charge-coupled device) are used. In this case, as a color coding filter to be used therein, various kinds of filters can be considered. An example of the color coding filters is such that two colors such as red and blue colors are arranged in a checker-board pattern. When the checker-board pattern color coding filter is used in an imaging device, a dot-sequential color signal is developed from the imaging device.
FIG. 1 shows a part of a color image pickup system in which two imaging devices are used. In the example of FIG. 1, in front of a first imaging device 1 there is located a green color (G) filter 2 and in front of a second imaging device 3 there is located a red-blue color filter 4 which consists of red (R) and blue (B) color filter elements arranged in a checker-board pattern. In this case, each of the colors of the color filter elements G, R and B of the filters 2 and 4 corresponds to the picture elements of the respective imaging devices 1 and 3, and the alignment or arranging pitch of the picture elements is selected as x in the horizontal direction and y in the vertical direction, respectively.
In this case, the two imaging devices 1 and 3 are shifted or deviated by x/2 in the horizontal direction with each other with respect to a focussed image. The reading out of signals from the respective imaging devices 1 and 3 are carried out in association with the television synchronization signal. Thus, from the imaging device 1 there is derived a green signal G which has frequency ahd phase characteristics shown in FIG. 2A. In this case, the sampling frequency is selected as 1/P.sub.x where where P.sub.x is the time which corresponds to a time in which the scanning of the television signal moves the distance or pitch x.
In FIG. 2A, the solid line represents the base band component of the green signal G and the one-dot chain line represents its side band component, respectively.
From the other imaging device 3 there are derived dot-sequential red and blue color signals R and B which have frequency and phase characteristics shown in FIGS. 2B and 2C, respectively.
Since the imaging devices 1 and 3 are shifted by x/2 as set forth above, the phases of the side band components are different by 180.degree. between the color signals G and B/R at the frequency 1/P.sub.x, where B/R represents the dot-sequential color signal of red and blue colors. FIGS. 2B and 2C are signals from the adjacent lines.
In the color coding filter 4, the red and blue color filter elements are arranged in a checker-board pattern as set forth above, so that the phase relation of the color signals B and R is inversed at the frequency of 1/2P.sub.x.
By way of example, a luminance signal S.sub.Y is synthesized from the color signals G, R and B along the following equation (1). EQU S.sub.Y =1/2{(G.sub.n +G.sub.n+1)+1/2(R.sub.n +R.sub.n+1)+1/2(B.sub.n +B.sub.n+1)}
The synthesis by the equation (1) is determined in view of the phase relations of FIGS. 2A to 2C. In case of a colorless or black and white object, the side band components of carrier frequencies 1/P.sub.x and 1/2P.sub.x are cancelled. In this case, for example, a white object, it is of course selected to satisfy the relation of G=B=R.
In case of a colored object, if there is a vertical correlation in colors, at least side band components of 1/2P.sub.x are cancelled and hence almost all of spurious signals, which will be otherwise mixed into the luminance channel, can be suppressed. However, if there is no color vertical correlation, it is impossible to suppress the generation of a quasi-signal (spurious signal).
For example, an object with a pattern shown in FIG. 3, i.e., its upper half being red and its lower half being blue will now be considered. In this case, it is sufficient that only side band components generated with the frequency 1/2P.sub.x as the center are cancelled.
When the above object is picked up by the above color image pickup system, only a red signal R.sub.n is derived from a line n and a blue signal B.sub.n+1 is derived from a line n+1. Thus, a luminance signal S.sub.Y' of this case is expressed as follows: EQU S.sub.Y' =1/4(R.sub.n +B.sub.n+1) (2)
As may be apparent from FIGS. 2B and 2C, the phases of the carriers with the frequency 1/2P.sub.x are the same and hence the carriers are emphasized. Thus, even though the vertical correlation is utilized, the side band components generated with the frequency 1/2P.sub.x as the center can not be cancelled and a spurious signal is reproduced as a dot-like pattern as shown in FIG. 4.